Back fire help

T

Truewakeboarder

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I picked up a 1975 cobalt for free. All works not bad but inside interior is shot. I started with changing the heads. But runs fine and revs fine out of gear. As soon as I get about 2000 RPMs the boat back fires from carb. Timing is good and all new plugs and wires. I rebuilt the Holley 2bl carb and found a new cap and rotor. Still back fires. I haven't adjusted the idle screws yet. They are set to factory 1 screw out. Where should I start? I sprayed carb spry to check for vac leaks but non found. Thanks. I have gotten a lot of info on this site and this is my first post. Thanks for any help.
 
Bondo

Bondo

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Re: Back fire help

Truewakeboarder said:
I picked up a 1975 cobalt for free. All works not bad but inside interior is shot. I started with changing the heads. But runs fine and revs fine out of gear. As soon as I get about 2000 RPMs the boat back fires from carb. Timing is good and all new plugs and wires. I rebuilt the Holley 2bl carb and found a new cap and rotor. Still back fires. I haven't adjusted the idle screws yet. They are set to factory 1 screw out. Where should I start? I sprayed carb spry to check for vac leaks but non found. Thanks. I have gotten a lot of info on this site and this is my first post. Thanks for any help.
Click to expand...

Ayuh,.... Welcome Aboard,... Generally speakin', backfires are caused by a lean condition,....

I'd say yer Carb still needs some work...
 
T

Truewakeboarder

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Re: Back fire help

It's a 302 and the one of the heads had cracked. I changed the fuel filter and clean fuel lines. Took out the anti siphoning valve and checked the fuel pump. Took it back out and you can hear it hesitate prior to the backfire. I'm going to be installing a electric fuel pump and new hose. It will do 2200 rpms @ 15 mph before backfiring. I just wanna go fast!
 
Howard Sterndrive

Howard Sterndrive

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Nov 5, 2008
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4,603
Re: Back fire help

which firing order are you using?

302's and 351's are weird -the intakes swap back and forth but the firing order needs to match the cam
so the firing order embossed on the intake manifold may not be right for your motor

consult the factory manual for your motor for correct firing order. If the engine is not original, try to determine what cam is in it.

however all that said - lean sneeze is common when the motor is not getting enough fuel
 
T

Truewakeboarder

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Messages
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Re: Back fire help

The motor is original and had 351 heads. 289, 302, and 351W heads will fit. So i put 302 heads on. I did the 190 and 170 firing order and would only run on the 190. I think we may have found the problem being the condenser is for a 302 not for the presto-lite distributor. Found that a 75 jeep is what i need to tell the parts house for.The condenser is just siting in the distributor and not secured. i have ordered the new condensers and points since both are for a 75 ford. I hope this works. Plugs are good not showing they are leans so I'm hoping its the condenser not grounded causing weak spark.
 
T

Truewakeboarder

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Sep 29, 2012
Messages
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Re: Back fire help

So replaced the ballast relay. Engine runs smoother and still popped. Plugs show lean. Closed choke 1/2 - 3/4 and no more pop. Looks like the carb is going to be taking a bath in carb dip. Got the boat to travel 30 mph @ 4000 rpm but still popping so I hope this works!!!!! I have replaced everything else.
 
C

captk

Petty Officer 1st Class
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Apr 4, 2008
Messages
252
Re: Back fire help

Have you screwed the cond down it has to b tight or it dont work. Did you check the points with a dwell meter or are you guessin you set them right with a FEELLER gauge.
 
T

Truewakeboarder

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Sep 29, 2012
Messages
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Re: Back fire help

All is fixed. Didn't know these 302 had a 351 cam in them. All is good and was wakeboarding and doin 45 mph with no problem. Thanks for all the help
 
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